Triggered spark gap device

ABSTRACT

A triggered spark gap device as disclosed which is adapted to switch exceedingly high values of voltage and current over a large dynamic operating range. The device comprises first and second main electrodes defining a main gap and an auxiliary electrode defining a trigger gap with one of the main electrodes. The auxiliary electrode is electrically isolated from the main electrode as a separate discharge path and means are provided to limit the discharge current therethrough thereby minimizing deterioration of the auxiliary electrode under the influence of high-discharge current. Additionally, the auxiliary electrode is provided with means for preventing induced circulating currents to avoid excessive heating and deterioration thereof. In a preferred embodiment the dynamic range is extended by biasing the auxiliary electrode with reference to one of the main electrodes so that it may be positioned closer to the other main electrode to obtain reliable firing at low-main gap voltage. Additionally, the other main electrode may be provided with an auxiliary electrode for electric field forming to increase the upper limit of the dynamic range.

United States Patent [72] Inventor David R. Matthews Ann Arbor, Mich. [211 Appl. No. 865,420 [22] Filed Oct. 10, 1969 [45] Patented Dec. 28, 1971 [7 3] Assignee Laser Systems Corporation Ann Arbor, Mich.

[541 TRIGGERED SPARK GAP DEVICE 18 Claims, 3 Drawing Figs.

[52] U.S. Cl 315/241, 313/198, 315/335 [51] Int. Cl.. H05b37/00, 1101 j 17/00 [50] Field otSearch 313/197, 198, 204, 205; 315/60, 168, 234, 239, 241, 330, 335; 336/73 [56] References Cited UNITED STATES PATENTS 2,820,142 l/l958 Kelliher 336/73 X 3,328,632 6/1967 Robinson 313/198 X Primary Examiner-Roy Lake Assistant Examiner-Lawrence J. Dahl Attorney- McGlynn. Reising. Milton and Ethington ABSTRACT: A triggered spark gap device as disclosed which is adapted to switch exceedingly high values of voltage and current over a large dynamic operating range. The device comprises first and second main electrodes defining a main gap and an auxiliary electrode defining a trigger gap with one of the main electrodes. The auxiliary electrode is electrically isolated from the main electrode as a separate discharge path and means are provided to limit the discharge current therethrough thereby minimizing deterioration of the auxiliary electrode under the influence of high-discharge current. Additionally, the auxiliary electrode is provided with means for preventing induced circulating currents to avoid excessive heating and deterioration thereof. In a preferred embodiment the dynamic range is extended by biasing the auxiliary electrode with reference to one of the main electrodes so that it may be positioned closer to the other main electrode to obtain reliable firing at low-main gap voltage. Additionally, the other main electrode may be provided with an auxiliary electrode for electric field forming to increase the upper limit of the dynamic range.

TRIGGERED SPARK GAP DEVICE This invention relates to are discharge device and more particularly to triggered spark gap devices adapted for high voltage switching.

A triggered spark gap device is useful as a high-voltage switch which provides an arc discharge through a gaseous medium as the conductive path of the switch. Such spark gap devices are capable of switching extremely high voltages and high current values by means of a relatively low power trigger voltage. A typical triggered spark gap device comprises a pair of main electrodes with a main gap there between and adapted to be connected across a source of high voltage. A trigger electrode is disposed adjacent one of the main electrodes with a trigger gap therebetween and a trigger voltage source is connected thereto. With the high voltage applied across the main electrodes, the trigger voltage is effective to produce a discharge across the trigger gap and thus initiate ionization of the main gap whereby an arc discharge is established between the main electrodes to close the switch. The switch is opened by extinguishing the arc discharge between the main electrodes, suitably by removal of the high voltage thereacross.

There are many applications in which the triggered spark gap device is useful for switching exceedingly large values of peak power. The device is capable of switching voltages in excess of twenty thousand volts with currents in excess of one thousand amperes with a switching time on the order of one microsecond or less. Heretofore, triggered spark gap devices having such power handling capabilities have been very expensive to manufacture and of relatively short operating lifetime, measured in terms of tens or hundreds of cycles. Furthermore, the devices have been incapable of operation at switching frequencies in excess of a few cycles per second. For some applications a further shortcoming of the prior art devices is the relatively small dynamic range i.e., the range of voltage across the main electrodes which the device will reliably control with a given value of trigger voltage.

A triggered spark gap device with suitable operating characteristics is especially useful as the switching device in the electrical power supply for a high power gas laser. In such an application where the gas laser, for example a carbon dioxide pulsed laser, is pumped by direct electrical excitation a high voltage, high current impulse must be applied across the plasma tube, desirably with a current pulse rise time of a small fraction of a microsecond and a pulse duration on the order of a few microseconds. For this purpose a capacitor discharge power supply may be employed herein a storage capacitor is charged from a high voltage DC source and a pulse transformer is connected with its primary winding across the capacitor through a triggered spark gap device, the transformer secondary being connected across the laser plasma tube. The trigger electrode of the spark gap device is connected with a timed trigger pulse generator so that the laser may be pumped at a predetermined frequency by firing the spark gap device to discharge the capacitor through the pulse transformer thereby producing an exceedingly high voltage impulse across the laser plasma tube. Such an application of a triggeredspark gap device may require the spark gap to switch 20,000 volts at a current of one L000 amperes at a repetition rate of several hundred cycles per second with a switching time of less than one microsecond an an on-time or conduction interval of several microseconds. A pulsed gas laser which may utilize such a power supply is disclosed and claimed in U. S. Pat. application, Ser. No. 844,092 entitled Pulsed Gas Laser" and filed on July 23, 1969 by David R. Matthews and assigned to the same assignee as the subject invention.

Heretofore, triggered spark gaps have not been commercially available with the desired capability for power handling, switching repetition rate, lifetime and extended dynamic range.

Accordingly, an object of this invention is to provide a triggered spark gap device capable of switching exceedingly high values of voltage and current at a high switching rate and a greatly improved useful lifetime. A further and related object is to provide such a device which is economical to manufacture.

A more particular object of the invention is to provide a triggered spark gap device having a large dynamic range of operation, i.e., a large range voltage across the main electrodes which the device will reliably control with a given value of trigger voltage.

A further object is to provide a triggered spark gap device wherein the main electrodes may be constructed of one material especially adapted for carrying the main discharge current and field forming electrodes may be constructed of another material which is less expensive and easier to form without subjecting the latter to accelerated deterioration under the influence of high current discharge.

A further object of the invention is to provide a triggered spark gap device with field forming electrodes separate and electrically isolated from the main discharge electrodes thereby minimizing deterioration by erosion such as from ion bombardment or excessive heating by circulating currents.

An additional object of the invention is to provide a selectively triggered spark gap device in which a field forming electrode is disposed adjacent one of the main electrodes with limited discharge current therefrom by reason of a current limiting resistance in circuit therewith.

An additional object of the invention is to provide a triggered spark gap device including an auxiliary electrode which functions as a field forming electrode and as a trigger electrode.

in accordance with this invention there is provided a triggered spark gap device with first and second main electrodes defining a main gap and an auxiliary electrode adjacent the first main electrode and defining a trigger gap therewith. Means are provided to impart a certain degree of electrical isolation between the auxiliary electrode and the first main electrode whereby the electrodes have separate discharge paths, and the discharge current through the auxiliary electrode may be limited. The means for limiting the discharge current through the auxiliary electrode may take the fonn of a resistive means connected between the auxiliary electrode and the main electrode. in such an arrangement the resistive means may be used to hold the auxiliary electrode at different potential from that of the first main electrode, thereby enabling close spacing to the other main electrode with reliable firing at low main gap voltage and an extended dynamic range. In a preferred embodiment, the potential difference between the auxiliary electrode and the first main electrode is provided by a bias circuit for the auxiliary electrode such as a voltage divider connected with the main supply voltage. Additionally, the other main electrode may be provided with a second auxiliary electrode for electric field forming to increase the static breakdown voltage and thereby extend the dynamic range. Both auxiliary electrodes may be provided with means to limit the discharge current therethrough thus minimizing the deterioration thereof by erosion and excessive heating.

A further feature is the provision of means for preventing circulating currents through the auxiliary electrodes by using a slot therein avoid formation of a closed turn around the main gap. Thus, the auxiliary electrode may be formed of high conductivity metals which are relatively soft and easily worked.

A more complete understanding of the invention may be obtained from the detailed description which follows taken with the accompanying drawings in which FIG. I is a sectional view of the inventive spark gap device.

FIG. 2 shows a detail of construction; and

FIG. 3 is a schematic diagram of the inventive spark gap device in a switching circuit.

Referring now to the drawings, there is shown an illustrative embodiment of the invention in a triggered spark gap device which is especially adapted for use as a high-voltage switch in a pulse forming power supply such as that used for energizing or pumping a gas laser. in such an application the voltage to be switched may be upwards of 20,000 volts and the peak current values may exceed 1,000 amperes. The switching time must be a fraction of a microsecond, and the conduction interval may extend for several microsecond with a switching repetition rate of two hundred pulses per second or greater. Obviously, such a switching device must be of heavy duty construction adapted to dissipate large amounts of power and exhibit a high degree of reliability. In some applications it is desirable to provide such a switching device with adjustable electrodes, as will be illustrated, to permit optimum operation for given operating conditions.

As shown in FIG. 1, the triggered spark gap device comprises an envelope which encloses the cathode assembly 12 and the anode assembly 14 in a gaseous atmosphere with a main gap or discharge path therebetween. It will be appreciated as the description proceeds that either of the main electrodes may serve as the cathode or anode depending upon the desired circuit application.

The envelope 10 comprises a cylindrical sleeve 16 of insulating material and an end plate, or header 18 of metal for closing the envelope at one end and supporting the cathode assembly 12. The envelope also includes an end plate 20 of metal disposed at the other end of the envelope for supporting the anode assembly 14. The cylindrical sleeve 16 is provided with a gas inlet port 22 and a gas outlet port 24 to permit forced circulation of a gaseous atmosphere through the envelope in the discharge path between the main electrodes thereby providing a continuously cooled gaseous discharge medium. Alternatively, the envelope 10 may be hermetically sealed and filled with a suitable discharge gas at a desired pressure, as is well understood in the art. For operation at exceedingly high switching voltage and currents, the end plate 18 is provided with coolant passage 26 and the header or end plate 20 is provided with a similar coolant passage 28.

The cathode assembly 12 comprises a cathode terminal post 30 which extends through the end plate 18 and supporting main or cathode electrode 32. The cathode electrode is suitably of simple rodlike configuration with a flat end and constructed of a refractory metal such as tungsten, tantalum, molybdenum or the like. The terminal post 30 is constructed of metal and is provided with an axial bore adapted to receive the electrode 32 in a press fit engagement. The terminal post 30 includes a head 34 adapted for connection to an external electrical circuit. It also includes a threaded shank 36 in threaded engagement with the end plate 18 and inner shank portion 38 for purposes to be described. Thus, the terminal post 30 provides for axial positioning of the cathode electrode 32 by reason of the screw thread mounting thereof in the end plate 18. When such a mounting is employed it may be desirable to supplement the heat transfer relation between the post 30 and end plate 18 by means of a heat transmitting grease or by use of a suitable locknut or the like.

The cathode assembly 12 also includes an auxiliary electrode 40 of dome-shaped configuration which is disposed adjacent the cathode electrode 32 and supported by an insulating plate or disc 42 which extends between the rim 46 of the electrode 40 and a metal washer 44 secured to the inner shank portion 38 of the terminal post 30. The auxiliary electrode 40 is provided with a central aperture defined by inner edge 48 thereof and disposed opposite the end of the cathode electrode 32. The auxiliary electrode is further provided with a slot 50 extending from the rim thereof to the central aperture.

56 in which is mounted a terminal member 58, a slidably mounted conductive pin 60 and a conductive spring 62 ex tending therebetween and resiliently urging the pin 60 into engagement with the rim 46 of the auxiliary electrode 40. This arrangement permits continuous electrical contact between the auxiliary electrode 40 and the terminal member 58 during adjustment of the position of the cathode electrode. In the event that a fixed cathode position is used, the trigger terminal assembly 54 may comprise a single conductive member extending through the end plate 18 and insulated therefrom into engagement with the auxiliary electrode 40.

The auxiliary electrode 40 serves also as an electric field forming electrode and since it is separate from the cathode electrode it may be formed of a different material. For reasons which will appear more fully hereinafter, it may be constructed of a high conductivity easily formed metal such as copper or brass.

The auxiliary electrode 40 may be electrically connected to the washer 44 and hence terminal post 30 by an electrical resistor 52. The purposes of the foregoing features of the cathode assembly including the resistor 52 and the configuration of the electrode 40 will be described subsequently.

The auxiliary electrode 40 also serves as a trigger electrode and together with cathode electrode 32 provides a trigger gap extending from the inner edge 48 thereof to the tip of the cathode electrode 32. In order to supply a trigger voltage to the auxiliary electrode 40, a trigger terminal assembly 54 extends through the end plate 18 to the auxiliary electrode. The trigger terminal assembly 54 comprises an insulating bushing The anode electrode assembly 14 comprises an anode terminal post 64 which extends through the end plate 20 and is provided with an axial bore which receives the anode electrode 66 suitably in a press fit engagement. The anode electrode 66 is suitably of the same construction and material as the cathode electrode 32 previously described. The terminal post 64 is provided with a head portion 68 adapted for connection to an external electrical circuit an inner shank portion 72 and a threaded shank portion 70 which threadedly engaged the end plate 20. Thus, the anode 66 may be adjustably positioned by rotation of the terminal post 64 and is suitably held in position by a locknut 74. An auxiliary electrode 76, which serves as an electric field forming electrode, of dome-shaped configuration is disposed adjacent the anode electrode 66 and supported by an insulating disc 78 which extends between the outer rim 80 of the electrode 76 and a metal washer 82 secured to the inner shank portion 72 of the terminal post 64. The electrode 76 is provided with a central aperture defined by'an inner edge 84 and disposed opposite the anode electrode 66. The auxiliary electrode 76 also includes a slot 86 which extends from the outer rim 80 to the inner edge 84 in the same manner as auxiliary electrode 40. The auxiliary electrode 76 may be electrically connected through a resistor 88 to the washer 82 and thence to the anode terminal post 64. The purpose of the foregoing features of the anode assembly 14 including the electrical resistor 86 and the configuration of the auxiliary electrode 76 will be discussed subsequently. The auxiliary electrode 76 may be formed of a high conductivity metal such as copper or brass in the same manner as electrode 40 previously described.

In operation of the triggered spark gap device just described the high voltage supply source to be switched thereby is connected with its output terminals connected respectively to the cathode terminal post 30 and the anode terminal post 64. Thus, the high voltage thereof is impressed across the main discharge gap having a length a extending between the inner ends of the cathode electrode 32 and anode electrode 66. The trigger voltage source is connected with its output terminals extending between the trigger terminal member 58 and the cathode terminal post 30 thereby impressing the trigger voltage across the trigger gap of length 12 extending between the edge 48 of auxiliary electrode 40 and the inner end of the cathode electrode 32. The spacing of the cathode electrode 32 and the anode electrode 66 is adjusted to the value a" to establish the approximate value of the static breakdown voltage, i.e., the DC voltage level necessary to cause a discharge between the anode and cathode electrodes with no trigger voltage applied to the trigger electrode. For a given value of main gap length a the greatest dynamic range is established by adjustment of the auxiliary electrode 40 and the auxiliary electrode 76. These auxiliary electrodes are effective to influence the distribution of the electric field between the cathode and anode electrodes to avoid sharp changes in the field pattern, i.e., local regions of high-voltage gradient. For this purpose the auxiliary electrodes 76 and 40 have a spherical curvature and are adjusted in axial position with reference to the corresponding main electrode to obtain the desired field configuration.

The auxiliary electrode 76 serves primarily as a field forming electrode and to a certain degree it is electrically isolated from the anode electrode 64 so that they constitute separate conductive paths between the supply voltage terminals. An impedance in the form of resistor 88 is connected in series with electrode 76 and the electrical discharge current therefrom is limited. The electrode 76 is insulated from the anode electrode 66 by the insulating disc 78 and by the gap between the inner edge 84 and the electrode 66. However, the electrical resistor 88 provides a current conductive path between the anode electrode 66 and the auxiliary electrode 76. Thus, the potential of the auxiliary electrode 76 will follow (with a slight negative margin so long as any charging or leakage current flows) the potential of the anode electrode 66 until breakdown across the main gap occurs.

-In a similar manner, the auxiliary electrode 40 is to a certain degree electrically isolated from the cathode electrode 32. It is insulated from the cathode electrode 32 by means of the insulating disc 42 and the gap between the inner edge 48 and the cathode electrode 32. However, the auxiliary electrode 40 is connected by an electrically conductive path through the resistor 52 to the cathode electrode 32 and thereby follows (with a slight positive margin so long as any charging or leakage current flows) the potential of electrode 32 .until breakdown occurs.

In the arrangement just described the value of the cutoff voltage is established by the position of the auxiliary electrode 40 with respect to the cathode electrode 32. The cutoff voltage is the minimum value of voltage that can be applied between anode and cathode electrodes with reliable breakdown of the main gap upon the application of a given trigger voltage. It will be apparent that adjustment of the auxiliary electrode 40 will have the effect of varying the trigger gap length b and the distance between the auxiliary electrodes 40 and 76. This adjustment of the auxiliary electrode 40 must be correlated with the position of the auxiliary electrode 76 when the static breakdown voltage is being maximized so as to avoid the production of sharp changes in the electric field and, thus, avoid a reduction of the main static breakdown voltage.

With the supply voltage applied across the anode terminal post 64 and the cathode terminal post 30 and having a value between the static breakdown voltage and the cutoff voltage, the spark gap is in readiness for switching upon the application of a predetermined trigger voltage between the trigger electrode member 58 and the cathode terminal post 30. Such a trigger voltage will produce a breakdown across the trigger gap extending between the auxiliary electrode 40 and the cathode electrode 32 thereby producing ionization in the main gap between the anode electrode 66 and the cathode electrode 32. Consequently, the main gap breaks down with an are discharge between the anode electrode 66 and the cathode electrode 32 through the apertures in the auxiliary electrodes 76 and 40 respectively. The current discharge through the auxiliary electrode 76 is limited to a very small value by reason of the series resistor 88 and similarly the discharge current through the auxiliary electrode 40 is limited to a small value by the resistor 52. Consequently, the main discharge path is confined to a direct path between the anode and cathode electrodes 66 and 32 respectively. The induction of destructive circulating currents in the auxiliary electrodes 76 and 40 is prevented by reason of the slots 86 and 50 respectively. Without such slots the auxiliary electrodes would constitute closed turns around the main discharge path and circulating currents would be induced therein and overheat the electrodes. The slots, however, extend in a direction transverse to the direction of the induced electrical field and thereby obstruct the flow of currents.

It will now be appreciate that the inventive structure thus far described provides a triggered spark gap device in which the main electrodes are separate from the auxiliary electrodes and carry virtually all of the main gap current. By reason of the electrical isolation between the main and auxiliary elec trodes, the discharge current through the auxiliary electrodes is limited to a small value and thus the heating and erosion of the auxiliary electrodes is minimized. Furthermore, circulating induced currents from the main discharge current path are prevented by reason of the slots in the auxiliary electrodes. As a result, the arrangement permits the use of auxiliary electrodes constructed of high conductivity metal such as copper, or an alloy thereof, which is inexpensive and easily machined or otherwise formed.

The arrangement just described affords many advantages in terms of operating characteristics, economy of manufacture and lifetime of the triggered spark gap device, and it also enables the adjustment of the electrodes to obtain a wide dynamic range. However, there are applications in which it is desired to obtain an even greater dynamic range for the triggered spark gap device and for this purpose the device may be employed in an arrangement as set forth in FIG. 3.

Referring now to FIG. 3, there is shown an arrangement for substantially extending the dynamic range for the triggered spark gap device just described. As will appear from the description which follows, this is accomplished by the provision of a bias voltage arrangement for the trigger gap. In the embodiment of the invention in FIG. 3 the triggered spark gap device is illustrated schematically but may be of the same construction as that shown in FIG. 1 wherein like reference characters are used to designate the same parts. The spark gap device is utilized as a high-voltage switch in a high-voltage pulse power supply of the capacitor discharge type for energizing or pumping a gas laser. A direct current supply voltage is connected to the terminals and 92 of the power supply and a storage capacitor 94 is connected in series with the primary winding of a pulse transformer 96 across the supply voltage terminals. The secondary winding of the pulse transformer 96 is connected directly across the plasma tube electrodes of a laser 98. The spark gap device is connected across the supply voltage terminals in parallel with the storage capacitor 94 and the pulse transformer 96 and is adapted upon breakdown to discharge the storage capacitor through the transformer primary winding thereby generating a very high-voltage impulse in the secondary winding across the plasma tube of the laser. For this purpose the anode terminal post 64 is connected to the positive supply terminal and the cathode terminal post 30 is connected to the negative supply terminal or ground. In order to control the pulse repetition rate, a trigger voltage source is connected to primary terminal 100 of a trigger transformer 102 which has its secondary connected through a coupling capacitor 104 to the trigger terminal member 58 on the spark gap device. For he purpose of extending the dynamic range of the spark gap device, the auxiliary electrode 40 is biased to a predetermined voltage value with reference to the cathode electrode 32. This is suitably accomplished by a voltage divider arrangement including a resistor 106 and a potentiometer 108 connected across the supply voltage terminals with the movable contact of the potentiometer connected through a resistor 110 to the trigger terminal member 58.

In considering the operation of the embodiment of the invention as shown in FIG. 3, it will first be assumed that the electrodes are adjusted as described with reference to Flg. 1 so that the anode and cathode electrodes are spaced a distance a and the auxiliary electrode 40 and the cathode electrode 32 are spaced a distance b. If the movable contact of the potentiometer 108 is positioned at its lowermost point so that it is effectively connected to ground, the auxiliary electrode 40 is connected to the cathode electrode 32 through resistor 110. Accordingly, in this condition the spark gap device will be adjusted as in FIG. 1 and will exhibit substantially the same static breakdown voltage and cutoff voltage as described with reference to the operation of the embodiment shown in FIG. 1. Resistor 52 is not required in this arrangement.

The dynamic range of the embodiment of FIG. 3 may be extended by the provision for biasing the auxiliary electrode 40 with reference to the cathode electrode 32. With an applied voltage V across the supplyTerTninals 90 and 92 and hence across the anode and cathode electrodes 66 and 32 respectively, the auxiliary electrode 40 may be moved closer to the anode electrode 66 without affecting the static breakdown voltage provided that its bias voltage E potential is increased, i.e., made more positive. This is accomplished by displacement of the movable contact of potentiometer 108 in the positive voltage direction an amount commensurate with the displacement or adjustment of the auxiliary electrode 40. For example, if the bias voltage on the auxiliary electrode 40 is increased so that the ratio E/V is approximately equal to the ratio a/b, then the static breakdown voltage remains unchanged because the position of the auxiliary electrode 40 in the electric field between the anode and cathode electrodes corresponds to a voltage level which is equal to the bias voltage. In other words, the voltage gradient between the auxiliary electrode "40 and the anode electrode 66 remains substantially unchanged as the auxiliary electrode is moved through the electric field. It is to be noted, however, that the trigger gap length b increases with displacement of the auxiliary electrode 40 toward the anode electrode 66, thus requiring an increasing or highervalue of trigger voltage to insure trigger gap firing. As a practical matter, this is not a serious problem since the trigger gap does not require high power and high-voltage trigger sources are readily available.

Positioning of the auxiliary electrode 40 intermediate the anode electrode 66 and cathode electrode 32 with a bias voltage thereon as just described does not substantially affect the static breakdown voltage; it does, however, markedly increase the dynamic range by establishment of very low value of cutoff voltage. With the applied voltage V across the anode and cathode electrodes 66 and 32 respectively assume for explanatory purposes that the bias voltage E is approximately one-half of the applied voltage V. Further assume that the auxiliary electrode 40 is positioned approximately halfway between the anode electrode 66 and the cathode electrode 32, i.e., the trigger gap length b is approximately equal to one-half the main gap length a. When a trigger voltage is applied, an arc discharge will occur between the auxiliary electrode 40 and cathode electrode 32 and thereby electrically connect these electrodes together through the highly conductive discharge path so that they are at substantially the same voltage. In this condition the supply voltage V is applied directly between the anode electrode 66 and the auxiliary electrode 40 which are separated by a distance approximately equal to one-half a and, therefore, the voltage gradient is approximately twice as great as that between these electrodes before firing the trigger gap. Thus, it is now apparent that the applied voltage V may be reduced to a value determined by the spacing between the auxiliary electrode 40 and the anode electrode 66 instead of the spacing between anode electrode 66 and cathode electrode 32 as in FIG. 1.

Thus, it is seen that the embodiment of FIG. 3 substantially increases the dynamic range of the spark gap device. The upper limit of the range, i.e., the static breakdown voltage is largely determined by the length of the main gap between the anode electrode and the cathode electrode provided that the auxiliary electrode 40 is biased at a potential substantially proportional to the displacement toward the anode electrode. The lower limit of the range, i.e., the cutoff voltage is at a value determined largely by the distance between the anode electrode and the auxiliary electrode so that the gradient therebetween will produce reliable main gap discharge for a given value of trigger voltage.

Although the description of this invention has been given with respect to particular embodiments thereof, it is not to be construed in a limiting sense. Many variations and modifications of the invention will now occur to those skilled in the art. For a definition of the invention reference is made to the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A trigger spark gap device comprising: a first main electrode and a second main electrode in spaced relation and disposed in a gaseous medium, said electrodes defining a in a gap and being adapted to have a supply voltage applied thereacross, an auxiliary electrode disposed adjacent the first main electrode and defining a trigger gap therewith effectively adjacent said main gap between the main electrodes, resistive means connected between the auxiliary electrode and the first main electrode, and control means connected with the auxiliary electrode for selectively applying a trigger voltage to the auxiliary electrode to initiate ionization of the gaseous medium and produce current flow between the main electrodes under both dynamic and static supply voltage conditions.

2. The invention as defined in claim 1 wherein said auxiliary electrode is dome-shaped with a central aperture and a slot extending from said aperture to the rim thereof, the auxiliary electrode coaxially surrounding the first main electrode and being radially spaced therefrom by the central aperture.

3. The invention as defined in claim 1 and including a second auxiliary electrode disposed adjacent the second main electrode and having a central aperture therein in alignment with the main gap between the main electrodes.

4. The invention as defined in claim 3 wherein the second auxiliary electrode is dome-shaped and has a slot extending from the aperture therein its rim.

5. The invention as defined in claim 3 wherein the second auxiliary electrode is electrically connected to the second main electrode through a resistor.

6. The invention as defined in claim 1 wherein the main electrodes are formed of a refractory metal and the auxiliary electrode is formed of a nonrefractory, conductive metal.

7. A triggered spark gap device comprising: an envelope, a gaseous medium in said envelope, a first main electrode and a second main electrode disposed within said envelope in spaced relation a,supply voltage connected across the main electrodes, a dome-shaped auxiliary electrode having an aperture at the center thereof and being disposed surroundingly adjacent the first main electrode with the aperture opposite the first main electrode and defining a discharge path between the main electrodes, the auxiliary electrode being supported relative to said first main electrode by a support member of insulating material, an electrical resistor connected between said auxiliary electrode and the first main electrode, the auxiliary electrode and the first main electrode defining a trigger gap adjacent said discharge path, and conductive control means extending through said envelope and connected with the auxiliary electrode for controlling the voltage differential between the auxiliary electrode and the first main electrode substantially independently of the supply voltage thereby to control the initiation of discharge therebetween.

8. The invention as defined in claim 7 wherein said support means comprises .an insulating plate extending between the rim of the auxiliary electrode and the first main electrode.

9. The invention as defined in claim 8 wherein said auxiliary electrode includes a slot extending from said aperture to said nm.

10. The invention as defined in claim 7 wherein the first main electrode is supported on an axially movable terminal post to adjust the length of the discharge path and wherein said conductive means includes a spring-loaded plunger en gaging the rim of the auxiliary electrode.

ll. The invention as defined in claim 7 including a second dome-shaped auxiliary electrode having an aperture centrally located therein and being disposed adjacent the second main electrode with the aperture aligned with the discharge path between the main electrodes, the second auxiliary electrode being supported relative to the second 12. A triggered spark gap device comprising: a first main electrode and a second main electrode in spaced relation and disposed in a gaseous medium, said electrodes defining a main gap and being connected with a supply voltage source, an auxiliary electrode disposed intermediate said main electrodes and defining a trigger gap with one of the main electrodes, bias means connected between said auxiliary electrode and said one of main electrodes for establishing a potential gradient between the auxiliary electrode and said one of the main electrodes under static voltage source conditions for controlling the initiation of discharge current between the main electrode and trigger voltage supply means connected between said auxiliary electrode and one of said main electrodes.

13. The invention as defined in claim 12 wherein said bias means includes a resistor connected between said auxiliary electrode and said one of main electrodes.

14. The invention as defined in claim 13 wherein said bias means also includes a voltage divider network connected across the supply voltage and variable voltage selection means connected between the voltage divider and the auxiliary electrode.

15. The invention as defined in claim 14 wherein said aux iliary electrode is dome-shaped with a central aperture aligned with said main gap. 0

16. The invention as defined in claim 15 wherein a second auxiliary electrode is disposed adjacent the other of said main electrodes and is dome-shaped with a central aperture therein aligned with said main gap and is electrically connected thereto through a resistor.

17. The invention as defined in claim 16 wherein at least one of said auxiliary electrodes has a slot extending from the aperture therein to its rim.

18. The invention as defined in claim 12 including a housing for supporting the first and second main electrode and adjuster means supporting at least one of said main electrodes relative to the housing to permit variation in the axial spacing main gap. 

1. A trigger spark gap device comprising: a first main electrode and a second main electrode in spaced relation and disposed in a gaseous medium, said electrodes defining a gap and being adapted to have a supply voltage applied thereacross, an auxilIary electrode disposed adjacent the first main electrode and defining a trigger gap therewith effectively adjacent said main gap between the main electrodes, resistive means connected between the auxiliary electrode and the first main electrode, and control means connected with the auxiliary electrode for selectively applying a trigger voltage to the auxiliary electrode to initiate ionization of the gaseous medium and produce current flow between the main electrodes under both dynamic and static supply voltage conditions.
 2. The invention as defined in claim 1 wherein said auxiliary electrode is dome-shaped with a central aperture and a slot extending from said aperture to the rim thereof, the auxiliary electrode coaxially surrounding the first main electrode and being radially spaced therefrom by the central aperture.
 3. The invention as defined in claim 1 and including a second auxiliary electrode disposed adjacent the second main electrode and having a central aperture therein in alignment with the main gap between the main electrodes.
 4. The invention as defined in claim 3 wherein the second auxiliary electrode is dome-shaped and has a slot extending from the aperture therein to its rim.
 5. The invention as defined in claim 3 wherein the second auxiliary electrode is electrically connected to the second main electrode through a resistor.
 6. The invention as defined in claim 1 wherein the main electrodes are formed of a refractory metal and the auxiliary electrode is formed of a nonrefractory, conductive metal.
 7. A triggered spark gap device comprising: an envelope, a gaseous medium in said envelope, a first main electrode and a second main electrode disposed within said envelope in spaced relation, a supply voltage connected across the main electrodes, a dome-shaped auxiliary electrode having an aperture at the center thereof and being disposed surroundingly adjacent the first main electrode with the aperture opposite the first main electrode and defining a discharge path between the main electrodes, the auxiliary electrode being supported relative to said first main electrode by a support member of insulating material, an electrical resistor connected between said auxiliary electrode and the first main electrode, the auxiliary electrode and the first main electrode defining a trigger gap adjacent said discharge path, and conductive control means extending through said envelope and connected with the auxiliary electrode for controlling the voltage differential between the auxiliary electrode and the first main electrode substantially independently of the supply voltage thereby to control the initiation of discharge therebetween.
 8. The invention as defined in claim 7 wherein said support means comprises an insulating plate extending between the rim of the auxiliary electrode and the first main electrode.
 9. The invention as defined in claim 8 wherein said auxiliary electrode includes a slot extending from said aperture to said rim.
 10. The invention as defined in claim 7 wherein the first main electrode is supported on an axially movable terminal post to adjust the length of the discharge path and wherein said conductive means includes a spring-loaded plunger engaging the rim of the auxiliary electrode.
 11. The invention as defined in claim 7 including a second dome-shaped auxiliary electrode having an aperture centrally located therein and being disposed adjacent the second main electrode with the aperture aligned with the discharge path between the main electrodes, the second auxiliary electrode being supported relative to the second
 12. A triggered spark gap device comprising: a first main electrode and a second main electrode in spaced relation and disposed in a gaseous medium, said electrodes defining a main gap and being connected with a supply voltage source, an auxiliary electrode disposed intermediate said main electrodes and defining a trigger gap with one of the main electrodes, bias means connected between said auxiliary electrode And said one of main electrodes for establishing a potential gradient between the auxiliary electrode and said one of the main electrodes under static voltage source conditions for controlling the initiation of discharge current between the main electrodes, and trigger voltage supply means connected between said auxiliary electrode and one of said main electrodes.
 13. The invention as defined in claim 12 wherein said bias means includes a resistor connected between said auxiliary electrode and said one of main electrodes.
 14. The invention as defined in claim 13 wherein said bias means also includes a voltage divider network connected across the supply voltage and variable voltage selection means connected between the voltage divider and the auxiliary electrode.
 15. The invention as defined in claim 14 wherein said auxiliary electrode is dome-shaped with a central aperture aligned with said main gap.
 16. The invention as defined in claim 15 wherein a second auxiliary electrode is disposed adjacent the other of said main electrodes and is dome-shaped with a central aperture therein aligned with said main gap and is electrically connected thereto through a resistor.
 17. The invention as defined in claim 16 wherein at least one of said auxiliary electrodes has a slot extending from the aperture therein to its rim.
 18. The invention as defined in claim 12 including a housing for supporting the first and second main electrode and adjuster means supporting at least one of said main electrodes relative to the housing to permit variation in the axial spacing between said main electrodes, thus, to vary the length of the main gap. 